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Long-Term Debt (Schedule Of Long-Term Debt) (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Nov. 30, 2017	Aug. 10, 2016	Jul. 31, 2012	Dec. 31, 2010
Debt Instrument [Line Items]
Principal amount		$ 2,715	$ 3,853
Unamortized Premium (Discount) and Debt Issuance Costs		(47)	(16)
Long-term debt, net		2,668	3,837
Long-term Debt, Current Maturities		0	710
Long-term Debt, Gross Excluding Current Maturities		2,715	3,165
Debt Instrument, Unamortized Premium (Discount) and Debt Issuance Cost, Net, Excluding Current Maturities		(47)	(38)
Long-term Debt, Excluding Current Maturities		$ 2,668	3,127
Senior Notes [Member] \| 3.60% Senior Notes Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2022
Stated interest rate (percent)		3.60%
Principal amount		$ 250	250			$ 250
Unamortized Premium (Discount) and Debt Issuance Costs		(1)	(1)
Long-term debt, net		$ 249	249
Senior Notes [Member] \| 3.60% Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2026
Stated interest rate (percent)		3.60%
Principal amount		$ 750	750		$ 750
Unamortized Premium (Discount) and Debt Issuance Costs		(9)	(10)
Long-term debt, net		$ 741	740
Senior Notes [Member] \| Loan related to tax-exempt waste disposal revenue bonds due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2027
Principal amount		$ 11	11
Unamortized Premium (Discount) and Debt Issuance Costs		0	0
Long-term debt, net		$ 11	11
Senior Notes [Member] \| 6 1/2% GO Zone Senior Notes Due 2029 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2029
Stated interest rate (percent)		6.50%
Principal amount		$ 100	100
Unamortized Premium (Discount) and Debt Issuance Costs		(1)	(1)
Long-term debt, net		$ 99	99
Senior Notes [Member] \| 6 1/2% GO Zone Senior Notes Due 2035 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2035
Stated interest rate (percent)		6.50%
Principal amount		$ 89	89				$ 89
Unamortized Premium (Discount) and Debt Issuance Costs		(1)	(1)
Long-term debt, net		$ 88	88
Senior Notes [Member] \| 6.5% 2035 IKE Zone Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2035
Stated interest rate (percent)		6.50%
Principal amount		$ 65	65				$ 65
Unamortized Premium (Discount) and Debt Issuance Costs		0	0
Long-term debt, net		$ 65	65
Senior Notes [Member] \| 5.0% 2046 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2046
Stated interest rate (percent)		5.00%
Principal amount		$ 700	700		$ 700
Unamortized Premium (Discount) and Debt Issuance Costs		(24)	(25)
Long-term debt, net		$ 676	675
Senior Notes [Member] \| 4.375% Senior Notes Due 2047 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2047
Stated interest rate (percent)		4.375%
Principal amount		$ 500	500	$ 500
Unamortized Premium (Discount) and Debt Issuance Costs		(9)	(9)
Long-term debt, net		$ 491	491
Senior Notes [Member] \| 3.50% Senior Notes Due 2032 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2032
Stated interest rate (percent)		3.50%
Principal amount		$ 250	250
Unamortized Premium (Discount) and Debt Issuance Costs		(2)	(2)
Long-term debt, net		$ 248	248
Senior Notes [Member] \| 4.625% Westlake Senior Notes 2021 [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2021
Stated interest rate (percent)		4.625%
Principal amount	$ 625	$ 0	625
Unamortized Premium (Discount) and Debt Issuance Costs		0	20
Long-term debt, net		$ 0	645
Senior Notes [Member] \| 4.625% Subsidiary Senior Notes 2021 [Member]
Debt Instrument [Line Items]
Debt instrument maturity	2021	2021
Stated interest rate (percent)	4.625%	4.625%
Principal amount	$ 688	$ 0	63
Unamortized Premium (Discount) and Debt Issuance Costs		0	2
Long-term debt, net		$ 0	65
Senior Notes [Member] \| 4.875% Westlake Senior 2023 Notes [Member]
Debt Instrument [Line Items]
Debt instrument maturity		2023
Stated interest rate (percent)		4.875%
Principal amount	$ 434	$ 0	434
Unamortized Premium (Discount) and Debt Issuance Costs		0	11
Long-term debt, net		$ 0	445
Senior Notes [Member] \| 4.875% Subsidiary 2023 Senior Notes [Member]
Debt Instrument [Line Items]
Debt instrument maturity	2023	2023
Stated interest rate (percent)	4.875%	4.875%
Principal amount	$ 450	$ 0	16
Unamortized Premium (Discount) and Debt Issuance Costs		0	0
Long-term debt, net		0	16
Senior Notes [Member] \| 4.625% Senior Notes Due 2021
Debt Instrument [Line Items]
Long-Term Debt, Gross, Current Maturities		0	688
Debt Instrument, Unamortized Premium (Discount) and debt issuance cost, net, Current Maturities		0	22
Long-term Debt, Current Maturities		$ 0	$ 710